Intracellular channels as a route for protein passage in the capillary endothelium of the shark brain

Abstract

Abstract Capillaries in the area postrema and other brain regions at the level of the obex were examined in Japanese sharks under the electron microscope with or without intravascular injection of horse‐radish peroxidase and histochemical incubation. The postremal capillaries were characterized by a relatively wide perivascular space filled with connective tissue elements including fine fibrils about 100 Å in diameter, and by endothelial fenestration; however, the space and fenestration were apparently less extensive than in rats and cats (Hashimoto, '66; Rivera‐Pomar, '66; Hashimoto and Hama, '68). In other brain regions, the capillary endothelium showed peroxidase‐filled intracellular channels that connected the luminal surface with the basement membrane soon after peroxidase injection. Many of the adjoining endothelial cells were linked by tight junctions. Perivascular astrocytic processes had long, tight contacts of about 125 Å in overall thickness. Discussion was focused on a possible endothelial route for passage of so‐called barrier‐free substances in the central nervous system of higher vertebrates.